1. Field of the Invention
The present invention relates generally to data communications, and more particularly, to a system and method for virtual circuit backup in a communication network.
2. Related Art
In the field of data communications, a modem is used to convey information from one location to another. Digital technology now enables other communication devices, such as data service units (DSU's) to communicate large amounts of data at higher speeds. The communication scheme employed by these devices generally adheres to a model, known as the Open Systems Interconnect (OSI) Seven-Layer model. This model specifies the parameters and conditions under which information is formatted and transferred over a given communications network. A general background of the OSI seven-layer model follows.
In 1978, a framework of international standards for computer network architecture known as OSI (Open Systems Interconnect) was developed. The OSI reference model of network architecture consists of seven layers. From the lowest to the highest, the layers are: (1) the physical layer; (2) the datalink layer; (3) the network layer; (4) the transport layer; (5) the session layer; (6) the presentation layer; and (7) the application layer. Each layer uses the layer below it to provide a service to the layer above it. The lower layers are implemented by lower level protocols which define the electrical and physical standards, perform the byte ordering of the data, and govern the transmission, and error detection and correction of the bit stream. The higher layers are implemented by higher level protocols which deal with, inter alia, data formatting, terminal-to-computer dialogue, character sets, and sequencing of messages.
Layer 1, the physical layer, controls the direct host-to-host communication between the hardware of the end users' data terminal equipment (e.g., a modem connected to a PC).
Layer 2, the datalink layer, generally fragments the data to prepare it to be sent on the physical layer, receives acknowledgment frames, performs error checking, and re-transmits frames which have been incorrectly received.
Layer 3, the network layer, generally controls the routing of packets of data from the sender to the receiver via the datalink layer, and it is used by the transport layer. An example of the network layer is Internet Protocol (IP) which is the network layer for the TCP/IP protocol widely used on Ethernet networks. In contrast to the OSI seven-layer architecture, TCP/IP (Transmission Control Protocol over Internet Protocol) is a five-layer architecture which generally consists of the network layer and the transport layer protocols.
Layer 4, the transport layer, determines how the network layer should be used to provide a point-to-point, virtual, error-free connection so that the end point devices send and receive uncorrupted messages in the correct order. This layer establishes and dissolves connections between hosts. It is used by the session layer. TCP is an example of the transport layer.
Layer 5, the session layer, uses the transport layer and is used by the presentation layer. The session layer establishes a connection between processes on different hosts. It handles the creation of sessions between hosts as well as security issues.
Layer 6, the presentation layer, attempts to minimize the noticeability of differences between hosts and performs functions such as text compression, and format and code conversion.
Layer 7, the application layer, is used by the presentation layer to provide the user with a localized representation of data which is independent of the format used on the network. The application layer is concerned with the user's view of the network and generally deals with resource allocation, network transparency and problem partitioning.
The communications networks that operate within the OSI seven-layer model include a number of paths or links that are interconnected to route voice, video, and/or digital data (hereinafter, collectively referred to as "data") traffic from one location of the network to another. At each location, an interconnect node couples a plurality of source nodes and destination nodes to the network. In some cases, the sources and destinations are incorporated in a private line network that may include a series of offices connected together by leased-lines with switching facilities and transmission equipment owned and operated by the carrier or service provider and leased to the user.
This type of network is conventionally referred to as a circuit-switching network. Accordingly, a source node of one office at one location of the network may transmit data to a destination node of a second office located at another location of the network through their respective switching facilities.
At any given location, a large number of source nodes may desire to communicate through their respective switching facilities, or interconnect node, to destination nodes at various other locations of the network. The data traffic from the various source nodes is first multiplexed through the source switching facility, then demultiplexed at the destination switching facility, and finally delivered to the proper destination node. A variety of techniques for efficiently multiplexing data from multiple source nodes onto a single circuit of the network are presently employed in private line networks. For instance, time division multiplexing ("TDM") affords each source node full access to the allotted bandwidth of the circuit for a small amount of time. The circuit is divided into defined time segments, with each segment corresponding to a specific source node, to provide for the transfer of data from those source nodes, when called upon, through the network.
Other data communications systems, in contrast, have not been as successful with employing multiplexing techniques to enhance network efficiency further. In particular, frame-relay networks offer far fewer alternatives than their circuit-switching network counterparts. Frame-relay networks are often referred to as packet-switching networks. Packet-switching networks, as opposed to circuit-switching networks, allow multiple users to share data network facilities and bandwidth, rather than providing a specific amount of dedicated bandwidth to each user, as in TDM. Instead, packet switches divide bandwidth into connection less, virtual circuits. Virtual circuits can be permanent virtual circuits (PVC's) or switched virtual circuits (SVC's). As is known, virtual circuit bandwidth is consumed only when data is actually transmitted. Otherwise, the bandwidth is not used. In this way, packet-switching networks essentially mirror the operation of a statistical multiplexer (whereby multiple logical users share a single network access circuit). Frame relay generally operates within layer 2 (the data link layer) of the OSI model, and is an improvement over previous packet switching techniques, such as the industry standard X.25, in that frame relay requires significantly less overhead.
In frame relay networks, as in all communication networks, network outages are compensated for by providing some manner of backup. For example, if a particular circuit fails, an alternative circuit may be created to transport the data that can no longer be transported on the primary connection.
Typically, a router is relied upon to detect and react to network failures, and it is common for routers to perform this function in a similar manner for both circuit switched and non-circuit switched connections. Failure detection in a frame relay network is inherently more complex since failures can be sensed at both the physical level and the logical level. The frame relay logical management interface (LMI) protocol provides for the detection and notification of failures of both the entire link as well as individual circuits carried by that link. It is common for routers to disregard some or all of the possible failure notifications, to fail to detect those conditions in a timely manner (such as awaiting an LMI time-out condition following a physical failure indication) and to react to those conditions in a manner which is not optimized for a frame relay network. For example, the rerouting of all circuits upon the failure of a single circuit.
One problem with current systems is that physical link failures (layer 1 of the OSI model) alone may not trigger the establishment of a backup path. Equally, an LMI failure may not trigger the establishment of a backup path or may do so only after the LMI protocol timers have expired (often 40 seconds or more).
In addition, once a failure is detected and a backup path is established, all the primary data traffic will be routed on the backup path, even if only a partial fault (such as the failure of one virtual circuit) has occurred. For example, data transmitted over a frame relay network often suffers only a partial fault, or a network failure at some intermediate point across which only a portion of the data passes.
Therefore, it would be desirable to provide a system and method that will detect the failure of a physical link, as well as the failure of a logical link, in a frame relay network and perform backup based upon the physical failure, and furthermore, that will selectively configure the establishment of a backup circuit and restore the primary circuit based upon the particular virtual circuit failure that occurs.